1. Field of the Invention
This invention relates to an apparatus for determining the intensity of magnetic fields, in which Faraday rotation is observed using a magneto-optic element.
2. Description of the Prior Art
In recent years, as an optical method for determining the intensity of magnetic fields, the use of the Faraday effect has been proposed by, for example, Kyuma et al., IEEE, QE-18, 1619 (1982).
Methods for measuring the magnetic field intensity around a conductor through which current flows to thereby detect the current are advantageous in that an excellent insulation can be attained because of the use of light as a medium, and electromagnetic induction noise immunity may also be attained, so that the said methods can be applied to the disposition of power transmission.
FIG. 1 shows the principle of the method of measuring a magnetic field using the Faraday effect, in which a magneto-optic element 1 is placed in a magnetic field H. The linearly polarized light by a polarizer 2 is passed through the magneto-optic element 1. The polarization plane of light is rotated by an angle proportional to the magnetic field intensity H due to the Faraday effect. The polarized light rotated by the Faraday effect passes through an analyzer 3 which has a direction of polarization different from that of the polarizer 2 by 45.degree., and the angle of rotation, .theta., is converted to the change in the optical power. Optical output power in this case is given by the following equation. EQU Pout=K(1+sin 2.theta.) (1)
.theta.=CHl
wherein Pout denotes the optical output power, K is a proportional constant, .theta. is the Faraday rotation angle (degrees), l represents the length of the magneto-optic element 1 in the direction of the propagation of light, and C is the sensitivity constant in units of degrees/cm.Oe representing the sensitivity of the magneto-optic element.
Applications of magnetic field measurement apparatuses based on this principle have been proposed, such as that which detects zero-phase current to determine the occurrence of accidents by feeding signals from magnetic field measuring instruments arranged at multiple points to an arithmetic operation unit where the waveforms are added or subtracted to generate reference signals.
A typical magneto-optic element used in such a is magnetic field measurement apparatus is YIG crystal which is represented by the general formula Y.sub.3 Fe.sub.5 O.sub.12. However, as shown in FIG. 2, the sensitivity constant C of YIG changes greatly with temperature, showing an increase as large as 16% over a temperature ranging from -20.degree. C. to 120.degree. C. around the working temperature, resulting in the practical problem of great deviation in the measurement accuracy with the change of the ambient temperature. To eliminate this problem, a rare-earth iron garnet crystal, represented by the general formula Tb.sub.x Y.sub.3-x Fe.sub.5 O.sub.12 wherein x is limited to 0.3.ltoreq.x.ltoreq.0.9, is used for a magneto-optic element. An apparatus which uses this magneto-optic element has a remarkably improved measuring-accuracy in which the variation with temperature is .+-.1% over a temperature ranging from -25.degree. C. to 120.degree. C.
A rare-earth iron garnet crystal substituted with Bi has a large Faraday effect and, when used for a magneto-optic element, improves the sensitivity of the magnetic field measurement apparatus. At present, the Bi-substituted rare-earth iron garnet crystal having a temperature independent sensitivity constant and good characteristics in practical application, has not heretofore been available.
Because magneto-optic elements with a length of about 2 mm are required, rare-earth iron garnet crystals such as YIG which does not include Bi are made by the Flux method or the FZ method, which makes the manufacturing period long, causing a disadvantage in mass production of the measurement apparatus. Moreover, a magnetic field measurement apparatus utilizing as a magneto-optical element such rare-earth iron garnet crystals which do not include Bi must be provided with an expensive light source and photo-detector designed for the 1.3 .mu.m band, which makes the magnetic field measurement apparatus expensive.